1. Field of the Invention
The present invention relates to a lithium/sulfur accumulator comprising an electrode separator soaked with an excess quantity of electrolyte.
The invention can in particular be used in electric power storage.
2. Description of Related Art
Lithium accumulators or batteries are currently used as an autonomous power source, in particular in portable equipment. Due to their mass and volume energy density (from 160 to 240 Wh/kg; from 300 to 600 Wh/l), they tend to progressively replace nickel-cadmium (Ni—Cd) and nickel-metal hydride (Ni-MH) accumulators. Further, such systems have a lifetime capable of reaching 500, or even 1,000 cycles.
Lithium ion or Li-ion batteries have a structure containing at least one unit cell comprising two electrodes arranged on either side of a separator (organic or inorganic) soaked with an electrolyte comprising a lithium salt. The two electrodes, one positive (typically made of lithium cobalt oxide LiCoO2) and the other negative (graphite), are both assembled on a metal current collector.
Much research has also been carried out on lithium/sulphur (Li/S) accumulators where the positive electrode comprises a sulphur material. The development of Li/S accumulators is particularly based on the properties of elementary sulphur, which has 2,600 Wh·kg−1 of Li2S.
Sulphur is inexpensive, naturally plentiful, and has a low environmental impact. It thus is a highly promising positive electrode material for such Li/S accumulators where lithium reacts with elemental sulphur (S8) according to the following reaction:16Li+S8→8Li2S
The created potential difference is approximately 2.1 V (vs. Li+/Li), for a theoretical specific capacity of 1,675 mAh·g−1 of sulphur.
However, sulphur is an insulating material generally soluble in the organic electrolytes of accumulators. Further, its dissolution may corrode the negative Li electrode and thus cause a significant self-discharge of Li/S accumulators.
Further, during the discharge of a Li/S accumulator (FIG. 1), elemental sulphur (S8) is reduced by the metal lithium to form lithium polysulfide intermediate products of general formula Li2Sn (2<n<8). These more specifically are chains comprising sulphur atoms, negatively charged, associated with lithium ions, and soluble in organic electrolytes. The products present at the end of the reduction of elemental sulphur also comprise Li2S2 and Li2S. Such compounds, which are little or non-soluble in the electrolyte, may precipitate at the negative electrode. Due to their electronic insulator properties, they can thus cause the passivation of the negative electrode, its electric insulation, and the end of the discharge.
Lithium polysulfide intermediate products, Li2Sn, are also capable of reacting with the negative electrode (Li). They thus also promote the self-discharge. Further, they are responsible for the creation of a shuttle mechanism which occurs in charge and which adversely affects the accumulator performance, particularly in terms of coulombic efficiency. However, discharge product Li2S is non-soluble in the electrolyte and is electronically insulating. Its precipitation at the end of the discharge causes the passivation of the surface of the electrodes, which then become inactive. Thus, the capacity of a Li/S accumulator is generally in the range from 300 to 1,000 mAh·g−1 of sulphur while the theoretical capacity is 1,675 mAh·g−1 of sulphur.
The precipitation of the sulphur compounds depends on the nature of the electrolyte and on its quantity. Indeed, the nature of the electrolyte solvent may affect the mass storage capacity of the lithium sulphur accumulator (FIG. 2). Thus, the influence of the nature of the solvent on the electrochemical performance generally translates as a lengthening of the second discharge stage and thus an increase of the discharge capacity. This effect results from the solvation of the lithium polysulfide species. For example, solvents such as polyethylene glycol dimethyl ether (PEGDME) have a relatively long ether chain, which enables to significantly solubilize short-chain lithium polysulfides, thus limiting their precipitation.
Further, the quantity of electrolyte, and thus of solvent, to be used to prepare a lithium accumulator depends on the geometric structure thereof. Indeed, in the case of a button cell containing a single unit cell (formed of a positive sulphur electrode, of a negative metal lithium electrode, and of a single liquid organic electrolyte supported by a separator interposed between the two electrodes), an excess quantity of electrolyte is introduced. This excess quantity may be in the order of 500%, to fill empty spaces. Generally, the components have a total thickness close to 400 μm while the button cell has an approximate total thickness of 3 mm. This difference is compensated by the presence of a spring, which ensures a fine contact between all components and electrically connects the collector of the negative electrode to the upper cap. This type of accumulator thus has a significant bulk, relatively to the matter really engaged in the electrodes and the electrolyte. The excess electrolyte enables to overcome possible electrolyte losses in the dead volume, and problems of incomplete wetting of the electrodes and of the separator.
However, in the case of high-energy-density or high-voltage Li-ion accumulators, appearing in cylindrical or prismatic form or in the form of stacks of a plurality of unit cells, the quantity of electrolyte is adjusted to provide a complete wetting of the electrodes and of the separator, without for all this adding useless mass to the accumulator. Indeed, a superfluous quantity of electrolyte would not improve the accumulator performance, in terms of capacity, potential, or power response. However, it would increase the real mass of the accumulator and, for an identical electrochemical performance, would thus decrease the volume and mass storage densities. It is thus necessary to control and to optimize the quantity and the number of each component introduced in the battery. Typically, the quantity of electrolyte introduced into a Li-ion accumulator corresponds to the quantity necessary to wet the electrodes and the separators, plus approximately 10% to fill possible interstices present in the battery (other than those contained in the separator and the electrodes).
The study of button-cell type battery components involves an excess quantity of electrolyte, to fill up the dead volume of the cell. However, in the case of series cells, superfluous volumes and masses are decreased as much as possible, and the electrolyte is introduced by as limited a quantity as possible.
As already indicated, Li/S accumulators have many advantages and disadvantages, particularly as concerns the precipitation of polysulphur compounds, the electrode passivation, or the mass energy density.
Due to the dissolution of the species produced during the discharge and to their subsequent precipitation, the discharge mechanism of Li/S accumulators differs from that of conventional Li-ion technologies. The quantity and the nature of the introduced electrolyte directly affect the electrochemical performance. The conventional preparation mode of lithium batteries is thus not applicable to Li/S accumulators. A simple Li-ion accumulator separator, of microporous polyolefin separator type, filled with an excess quantity of electrolyte from 10 to 20%, is not satisfactory in the case of a Li/S system. Such very thin and moderately porous separators do not enable to receive a sufficient quantity of electrolyte for the proper operation of the redox process of Li/S accumulators.
Conventional Li-ion accumulators are generally developed in button-type cells before scaling up by transposition and adjustment of the geometry and of the components initially developed in a unit cell. However, this cannot be envisaged in the case of Li/S accumulators due to precipitation/passivation phenomena. Indeed, as illustrated in FIG. 3, a poor electrochemical performance would then be obtained for a Li/S accumulator directly transposed from a button cell architecture.
The Applicant has developed a lithium/sulphur accumulator having its electrode separator enabling to solve prior art technical problems relating to the passivation of electrodes by precipitation of the sulphur species, while maintaining a significant mass energy density.